An impact switch actuates in response to an acceleration having a magnitude that exceeds a predetermined acceleration threshold. Impact switches are widely used in military applications, such as safing-and-arming and/or detonation systems in munitions (e.g., artillery shells, missile warheads, armor-piercing projectiles, etc.), and non-military applications, such as damage monitoring systems for shipping containers, vehicle air bag deployment systems, and automatic seat belt tensioning systems.
Military applications present some rather unique challenges to the use of impact switches for acceleration detection. First, a munition, such as an artillery shell, must reliably distinguish acceleration due to the firing of the round (i.e., “setback” acceleration) from accelerations due to non-firing-related “environmental events,” such as incidental shock and vibration. The ability to distinguish between these accelerations mitigates the potential for accidentally induced detonation from accelerations that arise during handling and transport, by incoming enemy artillery rounds, etc.
Second, the munition must be able to reliably detect acceleration due to impact. Failure of a munition to detonate upon impact reduces the effectiveness of its launch system, endangering it and its associated personnel. Further, undetonated ordinance remains a hazard to human life and property at its landing site until the munition is removed, safely detonated or disarmed, which can be extremely expensive and dangerous.
Many approaches have been reported in the prior art for safing, arming, and detonating a munition. In some approaches, an impact switch arms a munition based solely on detection of setback acceleration, which is typically tens to thousands of G's in magnitude. In other approaches, setback acceleration is not detected but a spin-rate sensor or rotationally activated switch that senses or reacts to angular acceleration due to the spinning of a munition (hundreds to thousands of rotations per second (rps)) is used to arm the projectile. In some approaches, a munition is armed only when both setback and angular accelerations are detected. In most prior-art systems, a separate impact switch is used to detonate the munition at impact.
Numerous impact switches have been developed in the prior art. Simple mechanical impact switches include crush-switches, deformable switches, or spring-loaded fuze-type elements, such as those disclosed in U.S. Pat. Nos. 6,765,160, 4,174,666, 2,938,461, and 2,983,800. Unfortunately, such switches actuate in response to any acceleration that exceeds a magnitude threshold and, therefore, provide little or no protection from inadvertent actuation.
Damped-response impact switches have been developed to provide some discrimination between spurious accelerations and accelerations due to a launch event. In some prior-art switches, magnetic damping has been exploited to provide a damped switch response, such as switches disclosed in U.S. Pat. Nos. 7,289,009 and 7,633,362. In other prior-art switches, mechanical integrators or fluidic systems have been used to provide a damped switch response, such as is disclosed in U.S. Pat. Nos. 4,900,880, 5,192,838, 5,705,767, and 5,272,293.
Unfortunately, such prior-art impact switches have several disadvantages. First, attaining a proper level of damping has proven challenging. In addition, more complicated mechanical systems require precision assembly and fabrication, which significantly increases switch cost. Further, complicated mechanical systems are more prone to failure. Still further, a drive toward “smart weaponry” has made miniaturization of systems such as impact switches highly desirable and many prior-art approaches toward damped impact switches make miniaturization difficult, if not impossible.
An impact switch having a damped response that is inexpensive, reliable, and compact, therefore, would represent a significant advance in the state-of-the-art.